A magnetic resonance imaging (MRI) device is a medical diagnostic imaging device, which applies a radio frequency magnetic field and a gradient magnetic field to a subject placed in a static magnetic field, detects signals generated by nuclear magnetic resonance in the subject, and forms an image from the signals.
As a method capable of obtaining images in which cerebral infarction at an acute stage, tumors etc. are emphasized with high signal strength, there is the diffusion-weighted imaging in which imaging is performed with emphasizing diffusion of water molecules. Since cerebral infarction at an acute stage is in a state of cellular edema, such diffusion is suppressed. Further, such diffusion is also suppressed in tumors densely containing cells. Therefore, in the diffusion-weighted imaging, diffusion coefficients of these sites become smaller than those of other tissues, and signals from them are detected with higher strengths.
As a typical imaging technique for obtaining diffusion-weighted images, there is the diffusion-weighted echo planar imaging. This method is an ultra high-speed imaging technique based on the single shot echo planar imaging, additionally using an MPG (motion probing gradient, diffusion-weighted gradient magnetic field) pulse, which is a gradient magnetic field pulse for emphasizing the diffusion. Since the MPG pulse generally has a higher strength and is applied for a relatively longer time, eddy currents and vibrations thereby induced result in fluctuation of magnetic field, which eventually degrades image quality.
There is a technique for suppressing influence of eddy currents by measuring eddy currents generated upon applying MPG as gradient magnetic field components of the slice direction, and applying a gradient magnetic field so as to cancel the components at the time of main scan (refer to, for example, Non-patent document 1).
In that technique, signals are measured by using a pulse sequence for the diffusion-weighted echo planar imaging with read-out gradient magnetic field pulse strength and phase-encoding gradient magnetic field strength of zero. This measurement is performed for positions of a plurality of slices in a predetermined slice direction, and temporal change of the magnetic field induced by MPG is measured for each slice position. Then, the change of the magnetic field is fitted with a linear function to obtain temporal change of the static magnetic field and temporal change of the gradient magnetic field in the slice direction. The absolute term of the obtained linear function represents the static magnetic field component generated by MPG, and the first order term represents the gradient magnetic field component in the slice direction. At the time of the diffusion-weighted imaging, a static magnetic field and a gradient magnetic field of the slice direction are applied so as to cancel those static magnetic field component and gradient magnetic field component, and thereby suppress the eddy currents generated by MPG.
Moreover, there is also a method for suppressing artifacts by a post-processing using temporal change of the magnetic field induced due to eddy currents generated by MPG, which is obtained by the aforementioned technique, not applying a gradient magnetic field for canceling them at the time of imaging (refer to, for example, Non-patent document 2). In this technique, phase shift (phase offset) and distortion of k-space data are calculated from the static magnetic field components and the gradient magnetic field components measured as described above, and images are corrected by phase offset correction and gridding to eliminate the influences of the eddy currents induced by MPG.